1. Field of the Invention
The invention relates to fluidized catalytic cracking.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst which vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air, to restore catalyst activity and heat catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst recycles to the cracking reactor to crack more fresh feed. Flue gas from the regenerator may be treated to remove particulates or convert CO, and then discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the (FCC) process has been to all riser cracking and zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is the Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, in the Jan. 8, 1990 edition of the Oil & Gas Journal.
The product distribution from modern FCC units is good. The volume and octane number of the gasoline is satisfactory, and the light olefins produced are upgraded via sulfuric or HF alkylation to high quality alkylate.
Unfortunately, refiners are finding it more difficult to make gasoline of sufficient octane and meet new specifications in regard to oxygenates, aromatics and benzene in the fuel. Reduced limits on RVP (Reid Vapor Pressure) and gasoline endpoint limit the amount of butanes that can be added, further exacerbating the problem.
Many refiners will face a shortage of light olefins (needed to make ethers and/or alcohols), with no efficient way of making more.
Most options available to FCC operators have limited potential. Use of shape selective cracking additives, or large cracking catalyst containing such additives, appeared to have only limited potential to increase yields of light olefins.
Pyrolysis units or thermal crackers produce large amounts of olefins, but little gasoline. A high severity, shape selective cracking process is also available, but like its closely related pyrolysis processes it makes large amounts of olefins, but relatively small yields of gasoline, which is highly aromatic and low in octane.
A reasonable way to summarize the state of the art on maximizing yields of light olefins from gas oil and heavier feeds is to focus on three catalytic approaches:
1. FCC Cracking catalyst with ZSM-5 and large pore zeolite sharing matrix, with large amounts of ZSM-5 crystal.
2. FCC units with additive ZSM-5 catalyst, in limited amounts.
3. Production of cracked gas from gas oil over pentasil zeolites at high severity.
These approaches will each be reviewed in more detail hereafter, in the order presented above, which is roughly chronological order.
1. Large Pore+ZSM-5-Shared Matrix
U.S. Pat. No. 3,758,403, Rosinski et al, Catalytic Cracking of Hydrocarbons with Mixture of ZSM-5 and Other Zeolites, taught the benefits of adding ZSM-5 to conventional large pore cracking catalyst formulations.
Example 2 used a catalyst consisting of 5 wt % ZSM-5, 10 wt % REY, and 85% clay. When used to crack a gas oil, it produced 11.42 LV % propylene, and a total yield of alkylate and C5+ gasoline of 89.1 LV %.
Example 3 used a catalyst consisting of 10 wt % ZSM-5, 10 wt % REY, and 80% clay. Although the ZSM-5 content doubled, propylene yields increased from 11.4 LV % to only 13.6 LV %. The total yield of alkylate and gasoline declined slightly, from 89.1 LV % to 88.6 LV %.
U.S. Pat. No. 3,847,793, Schwartz et al, Conversion of Hydrocarbons with a Dual Cracking Component Catalyst Comprising ZSM-5 Type Material, had a slightly different approach. The ZSM-5, which could be in the same particle with the large pore zeolite, or in a separate additive, was used to convert olefins to aromatics. A riser reactor with an enlarged upper portion was used, along with injection of a coking fluid near the top of the riser, to deactivate the large pore catalyst while leaving the ZSM-5 catalyst active. Gasoline boiling range material could be injected into the top of the riser for conversion. Table 2 shows that this approach reduced the mono olefin content of an FCC gasoline from 14.0 wt % to 2.9 wt %. The discussion of Examples 2 reports that ZSM-5 was effective for converting propylene to aromatics over a wide range of catalyst silica-alumina ratios.
Based on '793, large amounts of ZSM-5 should efficiently convert propylene into aromatics. This would reduce light olefin production, and perhaps exacerbate problems of producing gasoline without exceeding aromatics and/or benzene specifications.
Based on '403, use of large pore cracking catalyst with large amounts ZSM-5 additive gives only modest increases in light olefin production. A 100% increase in ZSM-5 content (from 5 wt % ZSM-5 to 10 wt % ZSM-5) increased the propylene yield less than 20%, and decreased slightly the potential gasoline yield (C5+ gasoline plus alkylate).
Neither approach seemed useful for making large amounts of light olefins via the FCC process using cracking catalyst containing large amounts of ZSM-5.
A drawback to an "all in one" catalyst, e.g., REY+H-ZSM-5 in a matrix, is catalyst availability. Mixing zeolite types in the same catalyst makes it impossible to use all the large pore cracking catalysts available today. There are more than 100 types of large pore cracking catalyst, but essentially all of the these are made without any ZSM-5 content. Refiners need to be able to, e.g., shift from a large pore cracking catalyst with bottoms cracking activity and high metals tolerance to an octane catalyst which minimizes hydrogen transfer reactions. FCC operation changes all the time in most refineries, in response to shifting crude supplies and varying product demands, and refiners want to retain the ability to use all of the cracking catalyst available to be able to constantly fine tune their FCC operation. For this reason, the marketplace has decided that use of additive catalysts is the only viable commercial option for the use of ZSM-5 and other shape selective additives. Use of separate additive catalysts will be reviewed next.
2. ZSM-5 Additives
Because refiners must retain the ability to use the myriad types of commercially available large pore cracking catalyst available today, they usually add additive catalysts, with 10-50 wt %, more usually 12 to 25 wt % ZSM-5 in a amorphous support, to their FCC units. Such additives have physical properties which allow them to circulate with the large pore cracking catalyst.
U.S. Pat. No. 4,309,280 taught adding very small amounts of powdered, neat ZSM-5 catalyst, characterized by a particle size below 5 microns. Adding as little as 0.25 wt % ZSM-5 powder to the FCC catalyst inventory increased LPG production 50%. Small amounts of neat powder behaved much like larger amounts of ZSM-5 disposed in larger particles.
A good way to add a modest amount of ZSM-5 to an FCC unit is disclosed in U.S. Pat. No. 4,994,424, incorporated by reference. ZSM-5 additive is added to the equilibrium catalyst in a programmed manner so an immediate boost in octane number, typically 1/2-2 octane number, is achieved.
U.S. Pat. No. 4,927,523, incorporated by reference, taught a good way to add large amounts of ZSM-5 to a unit without exceeding wet gas compressor limits. Large amounts were added, and cracking severity reduced until the ZSM-5 activity tempered from circulating through the FCC unit for several days.
ZSM-5 additive has been used commercially for almost a decade, and is now a well accepted way to increase C3 and C4 olefin yields and gasoline octane, at the cost of some loss in gasoline yield.
Recent work on ZSM-5 additives has been directed at stabilizing it with phosphorus or making the additive more attrition resistant. Phosphorus stabilized ZSM-5 additive is believed to retain activity for a longer time. There may be some change in yield pattern, but none that we have been able to observe in commercial refineries. Phosphorus stabilization thus reduces the makeup rate of ZSM-5 additive required.
One drawback to use of ZSM-5 additive, even with phosphorus stabilization, is that refiners fear dilution of the large pore cracking catalyst by addition of large amounts of ZSM-5, say over 2 or 3 wt % ZSM-5 crystal, or use of more than 5 or 10 wt % additive, will reduce yields of light olefins and seriously impair conversion. Most refiners operate with significantly smaller amounts of ZSM-5 than the upper limits recited above.
Another concern is how well the unit will respond when pushed to make even more olefins. The consensus is that small amounts of ZSM-5 additive make large amounts of olefins in an FCC unit operating at low severity, but the increase in yields of light olefins attributable to ZSM-5 declines as severity increases. "Working at low severity we observe an increase in light olefinic compounds, mostly branched, in the C5-C6 range. At the same time we detect an increase in light branched alkanes and almost no effect on the aromatics and naphthenes contents. When the cracking occurs at higher temperature we observe an increase in the C7-C8 aromatics and naphthenes, but a much smaller increase in the lighter compounds." Effect of Operation Conditions on the Behaviour of ZSM-5 Addition to a RE-USY FCC Catalyst, M. F. Elia et al, Applied Catalysis, 73 (1991) 195-216, 202.
The poor response to unusually large concentrations of ZSM-5 was reported in '403, while Elia et al have shown the unfavorable response of ZSM-5 to high severity FCC operation.
Thus it seemed that ZSM-5 would be of most benefit to refiners when used in small amounts, preferably in FCC units operating at modest severity levels. Attempts to increase yields of light olefins by increasing ZSM-5 content would meet with only modest success, while going to high severity operation (a proven way to increase production of light olefins in FCC) would reduce the effectiveness of the ZSM-5 at increasing yields of light olefins.
It seemed that refiners could not expect to increase yields of light olefins from the FCC units by using conventional amounts of ZSM-5 additive. Higher severity operation would increase olefin yields (due to higher temperatures and conversion) but diminish the olefin yields attributable to ZSM-5.
We wondered what would happen at very severe conditions, at temperatures and conversions beyond those used in FCC.
Any heavy feed can be thermally cracked at extremely high temperatures to produce large yields of ethylene and other light olefins. The high temperatures needed to get high conversions also degrade the C5+ liquid products. Thermal cracking is a good way to make ethylene, but not to make gasoline.
An unusually severe catalytic route to cracked gas over USY and/or pentasil zeolites is reported in U.S. Pat. No. 4,980,053. Although more suited to a petrochemical plant than a cracking refinery, the approach represents an upper limit on conversion over pentasil zeolites, and for that reason is reviewed below.
3. High Severity Pentasil Conversion
U.S. Pat. No. 4,980,053, Zaiting Li et al, Production of Gaseous Olefins by Catalytic Conversion of Hydrocarbons, has examples of conversion of vacuum gas oil to more than 50 wt % cracked gas over zeolites ranging from pentasil, to USY, and mixtures. The process is basically a pyrolysis process, which uses a catalyst to operate at somewhat milder conditions than thermal pyrolysis processes.
Four catalysts were tested. Although precise catalyst formulations, and zeolite concentrations within each catalyst formulation, are not reported, the following information was:
Catalyst Zeolite
A "CHO" Pentasil+REY PA1 B "ZCO" USY PA1 C "CHP" Pentasil PA1 D Mixture of B & C (Mix. of USY & Pentasil)
Thus catalyst A or "CHO", corresponds to the approach of '403, Rosinski, which used ZSM-5 and Y zeolite in the same catalyst particle, while catalyst D represents something closer to the additive approach.
The examples were run at conditions much more severe than those used in catalytic cracking--580 C (1076 F), at a 1 LHSV, a cat:oil ratio of 5, and steam:hydrocarbon ratio of 0.3.
______________________________________ Catalyst A B C D ______________________________________ wt % of: cracked gas 52.0 51.2 54.0 55.6 propylene 11.61 17.39 21.56 21.61 butylene 15.64 14.47 15.64 15.09 C5-205 C 31.0 33.1 27.0 27.5 Conversion 93.3 90.3 87.6 89.1 ______________________________________
It is difficult to say too much about the results because the zeolite content of the catalysts is not specified. The patentees report that "the yields of gaseous olefins over catalyst C and D are higher than the others." As far as gasoline yields, and conversion, the mixture (D=mix of pentasil+USY) gives less conversion and less gasoline yield than a single particle catalyst (A=Pentasil+REY). Use of a mixture also reduced butylene yields slightly, as compared to single particle catalyst A.
Example 2 of '053 reports production of fairly aromatic gasolines, containing more than 50 wt % aromatics. This was to be expected from the high temperatures and severe conditions. The octane number of the gasoline was 84.6 (motor method). The di-olefin content of the gasoline was not reported.
These results show use of separate additives of pentasil zeolite can reduce conversion and butylene and gasoline yield, as compared to use of single particle catalyst with both types of zeolite in a common matrix, during pyrolysis processing.